1. Field of the Invention
The present invention relates to a power supply apparatus for supplying predetermined supply voltages to a traveling-wave tube used to amplify and oscillate a high frequency signal, and a high frequency circuit system which comprises the power supply apparatus.
2. Description of the Related Art
A traveling-wave tube, a klystron and the like are electron tubes for amplifying or oscillating a high frequency signal by means of the interaction between an electron beam that is emitted from an electron gun and a high frequency circuit. For example, as illustrated in FIG. 1, traveling-wave tube 1 comprises electron gun 10 for emitting electron beam 50, helix 20 which is a high frequency circuit for causing electron beam 50 emitted from electron gun 10 to interact with a high frequency signal (microwave), collector electrode 30 for capturing electron beam 50 delivered from helix 20, and anode electrode 40 for extracting electrons from electron gun 10 to guide electron beam 50 emitted from electron gun 10 into helix 20.
Electron gun 10 comprises cathode electrode 11 for emitting thermoelectrons, heater 12 for applying thermal energy to cathode electrode 11 for emitting thermoelectrons, and Welnelt electrode 13 for converging electrons to form electron beam 50.
Electron beam 50 emitted from electron gun 10 is accelerated by a potential difference between anode electrode 40 and helix 20 and introduced into helix 20, and travels through helix 20 while interacting with a high frequency signal applied to helix 20. Electron beam 50 exiting helix 20 is captured by collector electrode 30. In this event, helix 20 delivers the high frequency signal which has been amplified by the interaction with electron beam 50.
As illustrated in FIG. 1, power supply apparatus 70 for supplying a predetermined supply voltage to each electrode of traveling-wave tube 1 comprises helix power supply 71 for supplying a negative DC voltage (helix voltage Ehel) to cathode electrode 11 of electron gun 10 on the basis of the potential applied to helix 20, collector power supply 72 for supplying a positive DC voltage (collector voltage Ecol) to collector electrode 30 on the basis of the potential applied to cathode electrode 11, anode power supply 73 for supplying a positive DC voltage (anode voltage Ea) to anode electrode 40 on the basis of the potential applied to cathode electrode 11, and heater power supply 74 for supplying heater voltage Eheat, which is an AC voltage or a DC voltage, to heater 12 of electron gun 10 on the basis of the potential applied to cathode electrode 11. Helix 20 is generally grounded through a connection to the housing of traveling-wave tube 1.
Helix voltage Ehel, collector voltage Ecol, and anode voltage Ea are generated, for example, using a known inverter for boosting the supply voltage fed from the outside, a transformer, a known rectifier comprising a rectifier circuit and a commuting capacitor, and the like.
Discharge bleeder resistors R1, R2 are connected between cathode electrode 11 and helix 20 and between cathode electrode 11 and collector electrode 30, respectively, for discharging electric charges accumulated on commuting capacitors (not shown) when the supply voltage is not fed.
In traveling-wave tube 1 illustrated in FIG. 1, the amount of electrons emitted from cathode electrode 11 can be controlled by anode voltage Ea applied to anode electrode 40, and the power of the high frequency signal delivered from traveling-wave tube 1 can also be controlled by anode voltage Ea. For example, even when traveling-wave tube 1 is applied with a high frequency signal having constant power, a pulsed high frequency signal can be delivered from helix 20 if anode electrode 40 is applied with a pulsed voltage.
In this connection, Japanese Patent Laid-Open No. 2005-45478 describes an example in which an input signal (high frequency signal) applied to traveling-wave tube 1 is detected to adjust anode voltage Ea in accordance with the input power such that the output power is not saturated, thereby improving the power efficiency of the output signal.
In the aforementioned conventional power supply apparatus 70, even if the operation of the inverter that is connected, for example, to the primary side of a transformer contained in the rectifier is stopped, the potentials of helix voltage Ehel and collector voltage Ecol remain as they are unless electric charges accumulated on the commuting capacitor connected to the secondary side of the transformer are discharged using some method. Accordingly, high voltages are maintained though the operation of various power supplies is stopped for testing and maintenance of the traveling-wave tube, klystron and the like. For this reason, maintenance works must be started after these electric charges have been sufficiently discharged.
In this connection, since anode power supply 73 employed herein provides low current supply capabilities, remaining anode voltage Ea, if any, will not cause serious problems. Generally, a load resistor is disposed at an output terminal of anode power supply 73 for stabilizing anode voltage Ea, so that electric charges accumulated on the commuting capacitor are discharged through the load resistor when the operation of anode power supply 73 is stopped.
On the other hand, since helix power supply 71 and collector power supply 72 employed herein provide high current supply capabilities, discharge bleeder resistors R1, R2 are disposed as illustrated in FIG. 1 to discharge electric charges accumulated on the commuting capacitors through discharge bleeder resistors R1, R2. Resistors having relatively large resistances (approximately several MΩ) are used for discharge bleeder resistors R1, R2 in order to reduce the current which flows during operation.
However, in the configuration in which electric charges are discharged using discharge bleeder resistors R1, R2, the electric charges are discharged based on a time constant which is determined by the capacitances of the commuting capacitors and the resistances of discharge bleeder resistors R1, R2 contained in helix power supply 71 and collector power supply 72. This causes a problem that it takes a long time until helix voltage Ehel and collector voltage Ecol become sufficiently low after the operation of power supply apparatus 70 is stopped.
Also, since discharge bleeder resistors R1, R2 have large resistances as mentioned above, they consume a large amount of power even if a small current flows therethrough, thus leading to the need for a larger package size in order to ensure sufficient electric power resistance. This causes a problem that large areas are needed for mounting discharge bleeder resistors R1, R2 which are mainly used only for testing and maintenance.
For reducing the time taken to discharge the electric charges accumulated on the commuting capacitors, it is imagined that the output terminals of helix power supply 71 and collector power supply 72 will be short-circuited to the ground potential using ground rod 75, as illustrated in FIG. 1. However, incorporating ground rod 75 into power supply apparatus 70 creates the problem that a larger area for mounting apparatus 70 is required. In addition, since short circuiting the outputs of helix power supply 71 and collector power supply 72 to the ground potential by using ground rod 75 requires making contact with high voltage (several kV) sites, the safety involved in this work is reduced.